The water on a frog's face (蛙の面に水, kaeru no tsura ni mizu) is a training regime that Jiraiya adopted while training the Ame Orphans. This would require the four of them to wear frog suits while training outside in the endless rain of Amegakure. This was to teach the young orphans to remain calm in the face of all adversities.

With this, Jiraiya also created a "Hop-In" system which was to be used in the hideout. It consisted of four wooden cards with images of a frog on one side and the other side was completely red. Each of their names were also printed just below the cards. Whenever one member was out of the hideout they were to turn the card onto the side with the frog so that the others would be aware that they were not there, and while they were in the hideout, the card should be on the red side. Jiraiya explained that this was done as a precaution because Amegakure was still war torn and politically unstable.[1]

Trivia

'Water on a frog's face' (蛙の面に水, kaeru no tsura ni mizu) is a Japanese expression referring to staying calm no matter what happens to you, just like how a frog will remain calm, even if you sprinkle water on its face.